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STUDY OF IONOSPHERIC IRREGULARITIES DURING INTENSE MAGNETIC STORMS
Luiz Felipe Campos de Rezende
1, Eurico Rodrigues de Paula
2, Inez Staciarini Batista
3,
Ivan Jelinek Kantor
4and Marcio Tadeu de Assis Honorato Muella
5Recebido em 23 fevereiro, 2006 / Aceito em 21 maio, 2007 Received on February 23, 2006 / Accepted on May 21, 2007
ABSTRACT.The effects of two intense magnetic storms over ionospheric irregularities were analyzed using GPS signal scintillation data from the stations of S˜ao Lu´ıs (2.57◦S, 44.21◦W, dip latitude 1.73◦S) in the equatorial region, S˜ao Jos´e dos Campos (23.07◦S, 45.86◦W, dip latitude 18.01◦S) and Cachoeira Paulista (22.57◦S,
45.07◦W, dip latitude 18.12◦S) both under the Equatorial Ionization Anomaly (EIA), and S˜ao Martinho da Serra (29.28◦S, 53.82◦W, dip latitude 18.57◦S), located in
the South of Brazil. Total Electron Content (TEC) data for S˜ao Lu´ıs and S˜ao Jos´e dos Campos, were also analyzed. The analyzed storms occurred on October 28-31, 2003 and on November 7-11, 2004. Both storm periods presented two main phases. In the nights of 29/30 and 30/31 of October, during the two storm main phase, it was observed that TEC over S˜ao Jos´e dos Campos reached higher values than the TEC for the magnetically quiet day of October 10, due to the effect of eastward electric field prompt penetration to magnetic equator that intensified the EIA. Compared to a quiet day (Oct 10), scintillation in the GPS signal amplitude due to ionospheric irregularity, quantified by the scintillation index S4, was stronger for Cachoeira Paulista (under EIA) during the night of 30/31but not for the night of 29/30 and for S˜ao Martinho da Serra was stronger during the nights of 29/30 and 30/31. Scintillation for the nights of 29/30 and 30/31 at these two stations lasted longer than on October 10, reaching the post midnight time sector. During the November 7-11 storm, TEC kept the behavior of a quiet day except during days 10 and 11 (up to 9 UT), when a large TEC decrease was observed. The GPS scintillation, compared to the quiet day November 19, was larger at the equatorial station of S˜ao Lu´ıs during the nights of 7/8 and 8/9 and it was completely inhibited for the S˜ao Lu´ıs and S˜ao Jos´e dos Campos stations during the nights of 9/10 and 10/11, probably due to action of westward disturbance dynamo electric field penetration to equator.
Keywords: GPS, ionospheric scintillation, magnetic storm, TEC (Total Electron Content).
RESUMO.Os efeitos de duas tempestades magn´eticas intensas sobre irregularidades ionosf´ericas foram analisados usando dados de cintilac¸˜ao do sinal de GPS das estac¸˜oes S˜ao Lu´ıs (2,57◦S, 44,21◦W, inclinac¸˜ao magn´etica 1,73◦S) na regi˜ao equatorial, S˜ao Jos´e dos Campos (23,07◦S, 45,86◦W, inclinac¸˜ao magn´etica 18,01◦S)
e Cachoeira Paulista (22,57◦S, 45,07◦W, inclinac¸˜ao magn´etica 18,12◦S) ambas sob a Anomalia da Ionizac¸˜ao Equatorial (AIE) e S˜ao Martinho da Serra (29,28◦S,
53,82◦W, inclinac¸˜ao magn´etica 18,57◦S), localizada no sul do Brasil. Dados de CET (Conte´udo Eletrˆonico Total) tamb´em foram analisados. As tempestades analisadas
ocorreram em 28-31 de outubro de 2003 e em 7-11 de novembro de 2004. Ambos per´ıodos de tempestades apresentaram duas fases principais. Nas noites de 29/30 e 30/31 de outubro, durante as duas fases principais da tempestade, foi observado que o CET em S˜ao Jos´e dos Campos alcanc¸ou valores mais altos que do dia 10 de outubro que foi magneticamente calmo, devido ao efeito de penetrac¸˜ao instantˆanea ao equador magn´etico de campo el´etrico dirigido para leste e intensificando a AIE. Comparada a um dia calmo (10 de outubro), a cintilac¸˜ao na amplitude do sinal de GPS devido `a irregularidade ionosf´erica, representada pelo ´ındice de cintilac¸˜ao S4, foi mais forte em Cachoeira Paulista (sob a AIE) durante a noite de 30/31 mas n˜ao para a noite 29/30 e em S˜ao Martinho da Serra, durante as noites 29/30 e 30/31. A cintilac¸˜ao para as noites 29/30 e 30/31 nestas duas estac¸˜oes tiveram maior durac¸˜ao do que para a noite de 10 de outubro, alcanc¸ando o setor das horas ap´os meia noite. Durante a tempestade de 7-11 de novembro, o CET teve o comportamento de um dia calmo, exceto durante os dias 10 e 11 (at´e 09 UT), quando uma diminuic¸˜ao significativa de CET foi observada. A cintilac¸˜ao, comparada ao dia 19 de novembro que foi calmo, foi maior na estac¸˜ao equatorial de S˜ao Lu´ıs durante as noites de 7/8 e 8/9 e foi completamente inibida para as estac¸˜oes de S˜ao Lu´ıs e S˜ao Jos´e dos Campos durante as noites 9/10 e 10/11, provavelmente devido `a ac¸˜ao da penetrac¸˜ao ao equador de campo el´etrico do d´ınamo perturbado dirigido para oeste.
Palavras-chave: GPS, cintilac¸˜ao ionosf´erica, tempestade magn´etica, CET (Conte´udo Eletrˆonico Total).
authors (Basu et al., 2001a, b; de Paula et al., 2004) since io-nospheric irregularities cause scintillation in the GPS signal am-plitude and phase and can affect telecommunication systems, and magnetically quiet time scintillation pattern can be modified during storms. The storms also can affect drastically the CET (Lin et al., 2005). As there is a strong interplay between the magnetospheric, ionospheric and atmospheric processes, which are substantially modified during magnetic storms (Abdu, 1997; Abdu et al., 1991, 2003; Batista et al., 1991, 2006; Tsurutani et al., 2004) we present in the next sections a short description of their quiet and disturbed behavior.
During magnetic storms supersonic solar plasma emissions distort the magnetosphere (see Fig. 1), that is a cavity formed by the interaction of the solar wind with the Earth’s magnetic field. The magnetosphere has a long tail, that extends in the opposite direction to the Sun (Davies, 1990). According to Gonzalez et al. (1994), a magnetic storm occurs when a long-lasting interplane-tary convection electric field leads, through a substantial energi-zation in the magnetosphere-ionosphere system, to an intensified ring current sufficiently strong to exceed some key threshold of the quantifying storm time Dst index. Energy from the solar wind is transferred to the ionosphere-thermosphere-magnetosphere system, intensifying convection electric fields in the magne-tosphere and producing an enhancement of particles precipita-tion, and currents in the high latitude ionosphere. During magne-tically disturbed periods the magnetospheric shielding layer is not effective to shield magnetospheric electric fields which therefore penetrate directly to low latitudes The structure and dynamics of the thermosphere and ionosphere is globally affected due to the increase of ionospheric conductivity, the Joule heating and the ion drag in the upper atmosphere of high latitudes and the distur-bance dynamo gives origin to westward electric field that pene-trates to equatorial region that could last up to 30 hours after the
Figure 1– Earth magnetic field. Source: Davies (1990).
The disturbed magnetospheric electric fields that penetrate to equatorial ionospheric region affect drastically the prereversal peak that is an intensification of the vertical plasma drift around 18-21 LT (21 LT corresponds to 24 UT for the Brazilian region un-der investigation in this work). The prereversal peak is explained through the action of uniform neutral wind in the F region (see Fig. 2). According to Farley et al. (1986) the electric field Ez
generated by the F region dynamo(−U×B)is mapped to the conjugated E-region along magnetic field lines as an electric field Eθ directed to the equator. This electric field generates a
low latitude Hall current,Jθ φ, directed to west. A peculiar
situ-ation occurs at regions close to the day-night terminator. Due to the much larger dayside conductivity (as compared to the night-side), no current flows in the nocturnal E-region and consequently negative charge accumulates in the terminator and gives origin to anEφfield and to a currentJφφthat tries to cancelJθ φ(shown
in Fig. 2).Eφis then mapped back to the F region and it causes,
firstly, an upwardE×Bdrift of the plasma to higher altitudes
and soon after, a downward drift around 21 LT.
Figure 2– Simple model to explain the prereversal peak caused by a uniform wind U. Source: Kelley (1989).
the upward verticalE×Bplasma drift of the equatorial F layer.
As previously shown, the zonal electric field that exists in the equatorial ionosphere is directed to the east during day, creating an upwardE×B/B2vertical drift velocity. Soon after the
sun-set, this eastward electric field is intensified (prereversal peak) by the F region dynamo and the plasma from F region is uplifted to high altitudes. Meanwhile, the plasma from low altitudes quickly decline due decreasing of the intensity of incident solar radiation (Kelley, 1989). After lifting to high altitudes in the equatorial re-gion, the plasma starts a descent movement along magnetic field lines. This movement happens due to the action of gravity(g)and pressure gradient(▽p)forces. This phenomenon (the plasma elevation and the subsequent descent along magnetic field lines to low latitudes) is known as the fountain effect (see the scheme in Fig. 3), giving origin to the Equatorial Ionization Anomaly.
Figure 3– Appleton Anomaly scheme.
The upward vertical plasma drift in the equator after sunset
that gives origin to the prereversal peak, is the main factor res-ponsible for the plasma irregularity generation (Fejer et al., 1999). The irregularities in the electronic density causes a GPS signal to scintillate and the corresponding amplitude pattern which is elongated in the north-south direction on the grounds drift from west to east during magnetically quiet period (Kintner et al., 2001). The ionospheric scintillation can be defined as fluctuations in the amplitude or phase of a radio wave. As the ionospheric scintilla-tions are highly dependent of the upward vertical plasma drift in the equator driven by east-west electric fields, the penetration to equator of eastward (westward) electric field from magnetosphe-ric (disturbance dynamo) origin during storms can trigger (inhi-bit) them. The scintillation amplitude is dependent also from the background ionization (TEC).
METHODOLOGY
In this work the magnetic indices Dst (Disturbance Storm Time), Kp (planetarische Kennziffer) and AE (Auroral Electrojet) were used to analyze the intensity of the storms. Other parameters like TEC and S4scintillation index were used to study the magnetically
disturbed behavior of the plasma and of the ionospheric irregula-rities, respectively. TEC corresponds to the number of electrons contained in the column of unitary base that extends from Earth surface to a determined height in the atmosphere. This parame-ter is measured in TEC units (1 TECU = 1×1016electrons/m2).
The S4index is the normalized standard deviation of the GPS
Figure 4– Location of the GPS receivers: SCINTMON(⋆)and Allen-Osborne(N).
receivers for scintillation monitoring. Turbo Rogue is an opti-mized receiver for ionospheric TEC measurements that is able to track up to 8 satellites simultaneously. The SCINTMON receiver was implemented through an ISA GPS Card (GEC Plessey GPS Builder-2TM). It is able to sample simultaneously signals from up
to 11 satellites. The data are only collected from satellites with elevation angle higher than 10 degrees. GPS Wide Band Power (WBP) of L1 (1.57542 GHz) that is transmitted by GPS satellites is sampled at 50 Hz rate. The SCINTMON receivers data analyzed were for S˜ao Lu´ıs, S˜ao Jos´e dos Campos, Cachoeira Paulista and S˜ao Martinho da Serra stations and TEC data (from Allen-Osborne receivers) were analyzed for the S˜ao Lu´ıs and S˜ao Jos´e dos Cam-pos stations (see Fig. 4). The disturbed days parameters were
compared to those for quiet days. Prior and post-storm periods were analyzed.
RESULTS AND DISCUSSIONS
depen-dence of the ionospheric irregularities with the magnetic activity, what is the main topic of this paper. Magnetic storms can inhibit the irregularities during their period of occurrence or can trigger irregularities at any month of the year, even during a period when irregularities are not expected (de Paula et al., 2004). This beha-vior during storm is very complex because it depends of the hour of the storm commencement, of the season, and if the storm occurred in the recovery phase of another storm. During a magne-tic storm an eastward (westward) electric field of magnetospheric origin can penetrate to equator through direct penetration (dis-turbance dynamo) reinforcing (weakening) the normal eastward E layer during day and F2 layer from18-21 LT electric fields, trigge-ring (inhibiting) irregularities. Eastward electric field penetration during magnetic storms also can trigger irregularities during the midnight-sunrise time sector.
For the October 28-31, 2003 storm, the first main phase occurred around 0100 UT on 30 of October when Dst reached –345 nT, the second one (strongest) occurred around 2300 UT on the same day and reached –401 nT (see Dst index in Fig. 5). The November 7-11, 2004 strong magnetic storm also presented two storm main phases. The first one reached –373 nT at 07 UT on the day 8 and the other –295 nT around 10 UT on day 10.
In Figure 5 are plotted the magnetic indices AE, Kp, and Dst and the S4 indices for the stations of S˜ao Martinho da Serra, Cachoeira Paulista (close to S˜ao Jos´e dos Campos) and S˜ao Lu´ıs and TEC for S˜ao Lu´ıs and S˜ao Jos´e dos Campos, during the storm period of October 28-31, 2003. The S4 indices represent the larger values between all available GPS satellites with elevation angles larger than 40◦, for each one minute.
We observed in Figure 5 that TEC had different behavior com-pared to the reference quiet day of October 10, 2003. Over S˜ao Jos´e dos Campos, under the EIA, during the nights of October 29/30 and 30/31, TEC reached higher values than during the day of October 10, simultaneously with a TEC decrease at equatorial station of S˜ao Lu´ıs (see Fig. 5). This is an evidence of magne-tospheric eastward electric field prompt penetration to low latitu-des during this storm, intensifying the EIA, and moving plasma from equator to low latitudes.
During this storm and considering that only S4 larger than 0.2 represents GPS scintillation, it was observed (see Fig. 5) larger (except for Cachoeira Paulista in the 29/30 night before 02 UT) and longer lasting scintillations at Cachoeira Paulista and S˜ao Martinho da Serra on the nights 29/30 and 30/31 compa-red to the quiet night (Oct. 10/11). At S˜ao Lu´ıs, the equatorial station, the scintillation amplitude was smaller during the nights of 29/30 and 30/31. Normally at the Brazilian sector there is
no scintillation after midnight. Magnetic storms can trigger post midnight scintillations.
The same magnetic and ionospheric parameters of Figure 5 were plotted in the Figure 6 for the November 7-11, 2004 storm. For this storm the main phase maximum (Dst reached –373 nT) of the first storm occurred around 07 UT in November 8, when the ionospheric ionization was still low, and TEC had a quiet day behavior for both stations and presented a substantial decrease for days 10 (all day) and 11 up to 09 UT. Compared to the quiet day November 19, the scintillation increased for the night 7/8 (S4 = 0.4) and less significantly for the next night 8/9 (S4 = 0.2) at S˜ao Lu´ıs, but it was completely inhibited for S˜ao Lu´ıs and S˜ao Jos´e dos Campos stations in the following two nights 9/10 and 10/11 (see Fig. 6) when TEC values were extremely low, even though these nights were under the second main phase effects.
CONCLUSIONS
The effects of two intense magnetic storms (October 2003 and November 2004) on TEC and ionospheric scintillations measu-red from GPS receivers over equatorial and low latitude stations, relative to quiet time days, were analyzed. It is shown that large variation of these two parameters were observed during the oc-currence of such storms, what is following presented.
During the October 2003 disturbed period, the storm indu-ced eastward magnetospheric electric field that penetrated to the magnetic equator on the 29/30 and 30/31 nights and intensified the vertical plasma drift, increasing TEC values over the low la-titude station of S˜ao Jos´e dos Campos and decreasing over the equatorial station of S˜ao Lu´ıs, and creating favorable conditions for the irregularities to grow at the stations of Cachoeira Paulista (however no scintillation increase was observed for this station on the 29/30 night) and S˜ao Martinho da Serra that are located under the south EIA crest. On this storm, the scintillation occurred even after 03 UT for those 2 stations and at our longitude sector this behavior is observed only during storm time.
Figure 5– Total Electron Content (TEC), scintillation index (S4) and the magnetic indices Dst, Kp, and AE for the period of October 28-31, 2003.
action of the westward magnetospheric electric field generated by the disturbance dynamo (Fejer & Scherliess, 1995; Scherliess & Fejer, 1997) that penetrated to the magnetic equator and inhibited the prereversal plasma drift after sunset. The TEC was completely washed out at S˜ao Jos´e dos Campos, during November 10 and 11 up to 9 UT what contributed substantially for the scintillation inhibition.
ACKNOWLEDGEMENTS
The authors acknowledge Dr. M.A. Abdu for providing the Turbo Rogue ICS-4000Z, Allen-Osborne Associates data. Luiz Felipe Campos de Rezende is grateful to FAPESP for support to this work, under Process 2006/06585-7.
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NOTES ABOUT THE AUTHORS
Luiz Felipe Campos de Rezendeis currently M.Sc. student in Applied Computer in the National Institute for Space Research. He is specialist in Software Engineering from Universidade do Vale do Para´ıba in 1991 and he has a BS in Computer Science from Universidade de Taubat´e in 1986. He works at INPE with TEC (Total Electron Content) and ionospheric scintillation and its impact on GNSS (Global Navigation Satellite System) since 2002.
Eurico Rodrigues de Paulais Senior Research of Aeronomy Division at the National Institute for Space Research (INPE). He earned the Ph.D. degree from INPE in 1986 in Atmospheric and Space Sciences and developed postdoctoral studies in Utah State University, Utah, USA, in 1988 and in 1995-1997. He works in electrodynamics of the low latitude ionosphere where the main interest field is the study of the ionospheric plasma irregularities based on GPS and VHF radar data and empiric and theoretical computational models.
Inez Staciarini Batistais Senior Research of Aeronomy Division at the National Institute for Space Research (INPE). She earned the Ph.D. degree from INPE in 1985 in Atmospheric and Space Sciences and developed postdoctoral studies in Boston University, USA, in 1997. She works in ionospheric research where the main interest fields are the ionospheric electrodynamical processes and the equatorial ionosphere plasma irregularities and bubbles.
Ivan Jelinek Kantoris Senior Research of Aeronomy Division at the National Institute for Space Research (INPE). He earned the Ph.D. degree from Rice University – Houston, Texas in 1972 and developed postdoctoral studies in the Max-Planck-Institut f¨ur Aeronomie, Lindau am Harz, Germany, in 1991. He works in ionospheric research where the main interest field is the measurement of the total electron content of the ionosphere using GPS data.
